Deep Tremors Detected Along San Andreas Fault

Seismologists have detected non-volcanic tremors deep along the San Andreas Fault in central California, near the epicenter of a 7.8-magnitude earthquake that struck Fort Tejon in 1857.

Such tremors had previously only been recorded in subduction zones, such as Japan and the Pacific Northwest, where one tectonic plate dives beneath another. The San Andreas fault, on the other hand, forms what is known as a transform plate boundary zone, an area where two plates slide past each other.

Scientists identified 110 tremor events lasting four minutes or more in the Parkfield area of central California during a three-year search period that ended with the 6.5-magnitude San Simeon earthquake on December 22, 2003.

These tremors may signal an increased likelihood of earthquakes on the San Andreas fault. Scientists have long expected California's next big earthquakeof magnitude 8 or higherto occur on the San Andreas fault.

"Because these tremors occur directly below the epicentral region of the 1857 quake, and because this part of the fault is locked and could rupture again soon, it is possible that increases in tremor activity may signal times of increased likelihood for a large event in the area," said Robert Nadeau of the Berkeley Seismological Laboratory in California.

Nadeau is the lead author of the study, which is described in tomorrow's issue of the journal Science.

Chaotic Shaking

The San Andreas Fault, the boundary between the North American and Pacific tectonic plates, runs 800 miles (1,300 kilometers) along the California coast and extends 10 miles (16 kilometers) down into the Earth. It is like a master fault in an intricate network of smaller faults that branch from the San Andreas and join it.

The scientists detected the tremors at a depth of 12.5 to 25 miles (20 to 40 kilometers) below the San Andreas fault near Cholame in central California, some 15 miles (25 kilometers) southeast of the town of Parkfield.

"The tremors are shaking of the ground that differ from earthquakes in that they last for up to 20 minutes, compared to earthquakes which last for less than 30 seconds," Nadeau said. "Unlike earthquakes, the tremor shaking is chaotic."

The discovery marks the first time such deep, non-volcanic tremors have been reported on the San Andreas fault, suggesting that the deformation causing earthquakes may have deeper origins than previously thought.